الفهرس 
PATHOPHYSIOLOGYOnly 14 pages are availabe for public view
 |  | from | 100 |  |  |
| | | from | 100 | | |
Abstract
A number of factors have been identified as possible causes of residual hearing loss after cochlear implant surgery. These include acoustic trauma caused while drilling over the cochlea, mechanical damage of the neural epithelium after electrode insertion, alteration of cochlear fluid homeostasis, bacterial infection and cochlear fibrosis after a foreign body reaction to the electrode components, or a sequela of the initial inflammatory reaction (Kiefer et al., 2004). Some of these may be related to the placement of the cochleostomy, size or length of the electrode, penetration of the scala by the electrode, or contamination of the perilymph with blood and bone dust (Manrrique et al., 2002).
Lehnhardt was the first to describe a soft-surgery technique for cochlear implantations in 1993. The main focus has been to preserve neural structures and increase the effectiveness of electrostimulation. The possibility of functional conservation was first described by Hodges et al., (1997).
A pioneering first report on EAS was published in 1999 by von Ilberg’s team in (Frankfurt, Germany), demonstrating several points: first, that the preservation of low-frequency hearing after cochlear implantation is possible. Second that the central auditory system is able to combine the signal from acoustic hearing at low frequencies with direct electric stimulation of the cochlear nerve without disturbing interferences. Third, the combined EAS results in improvements in speech perception and a more natural sound perception.
This method combines residual acoustic hearing with or without a hearing aid and electrostimulation via cochlear implant on the same ear. To ensure bimodal stimulation remaining cochlear function has to be preserved at least to some extent leaving delicate cochlear structures, especially those in the apical low frequency regions intact. The concept of EAS is to stimulate basal cochlear regions electrically and leave the more apical portions free of an electrode carrier. As mentioned previously, these areas are responsible for low frequency hearing and contain still working structures that can be stimulated with or without a conventional hearing aid, depending on the extent of residual function. Special surgical techniques contribute to preserving cochlear function and should be applied in every cochlear implantation with residual hearing (Kiefer et al., 2003).
The audiologic candidacy criteria for EAS or hybrid technology vary somewhat across manufacturers. Low-frequency thresholds generally can range from 20 dB HL up to 60 dB HL through 750 Hz, and thresholds at 1000 Hz and above must generally exceed 60 to 70 dB HL. Preoperative speech perception criteria require that aided CNC monosyllabic word recognition in the ear to be implanted should not exceed 50%-60% correct. Following surgery, the average low-frequency hearing loss ranges from 10 to 20 dB, depending on the electrode array and the nature of the surgical technique (Gantz et al., 2005; 2006; Gstöettner et al., 2005; Kiefer et al,. 2005; Leutje et al., 2007; Skarzynski et al., 2006).
The aim of EAS surgery is to insert the electrode array one full turn, a region that corresponds to the 1,000 Hz region which defines the end of electric and the beginning of acoustic stimulation (Von Ilberg et al., 1999). Anatomically, a 360° insertion into the scala tympani corresponds to an insertion depth of approximately 20 mm measured from the round window membrane (Otte et al., 1978).
Such an implant consisting of a CI coupled with a hearing aid in the same ear has been called a hybrid or combined electrical and acoustic device and leads to improved word recognition (Gantz et al., 2005; Luetje et al., 2007).
We identified only 4 articles dealing with this subject, three were case series and only one randomized controlled trial. These papers included 87 cases (of sever sensorineural hearing loss with residual hearing) underwent cochlear implantation using soft surgery for preservation of residual hearing. 57 of them showed good residual hearing, 3 showed partial preservation and 27 showed complete loss of residual hearing.
Our results in this meta-analytical study (as demonstrated in forest plot) showed that there is evidence that favors the group using soft surgery technique (EAS group) in one randomized control article, but there was no significant difference As Z test here is less than 2. These results yielded by the search and analyzed by the meta-analytic software.
We concluded that, the preservation of low-frequency hearing after cochlear implantation is possible using a soft-surgical technique for cochlear implantations, the combined EAS results in improvements in speech perception particularly in the presence of background noise. But unfortunately, the available data represents a less than satisfactory source for evidence based conclusions and decisions.
Finally we recommended that, future efforts should concentrate on developing well designed controlled studies and should be on broad basis in order to provide more statistically valuable results and the necessity for the presence of an international data base center to ensure the availability and the accuracy of data for further studies.